In general, a cylinder internal-pressure sensor for an engine for detecting an internal pressure (combustion pressure) of a cylinder by being faced with a combustion chamber of an engine is known, but this type of cylinder internal-pressure sensor needs to be mounted independently with a highly hermetical structure in a through hole formed at a predetermined position of the cylinder. Thus, a cylinder internal-pressure sensor in which the cylinder internal-pressure sensor is configured having a ring shape and capable of being mounted integrally on an outer peripheral surface of a distal end portion of an ignition plug which is another functional component attached to the engine, so that the cylinder internal-pressure sensor can be mounted on the cylinder along with the ignition plug.
As a cylinder internal-pressure sensor configured having such ring shape, a combustion pressure sensor disclosed in Patent Literature 1, a pressure sensor disclosed in Patent Literature 2, and a spark plug incorporating a pressure sensor disclosed in Patent Literature 3 are known.
The combustion pressure sensor in Patent Literature 1 is a combustion pressure sensor having an insulating body formed around a center electrode and a washer member electrically connected to a side electrode and formed around the insulating body, and incorporated in an ignition plug, provided with a piezo-electric element formed of lithium niobate installed between the insulating body and the washer member and also in the vicinity of an ignition gap between the center electrode and the side electrode. Moreover, the pressure sensor in Patent Literature 2 is a pressure sensor joined to an attachment hole instead of a gasket of a spark plug, and this pressure sensor is configured such that a mounting surface of a housing is brought into pressure contact with a cylinder head, and a radiation fin is fastened to the housing. Furthermore, the spark plug incorporating a pressure sensor in Patent Literature 3 is provided with a seat portion facing a plug mounting surface provided in an internal combustion engine when being mounted on the internal combustion engine; an accommodating member incorporated in this seat portion and holding a plurality of piezo-electric elements at predetermined intervals in a peripheral direction of an inner wall surface of the seat portion; an electrode plate formed having a planar shape corresponding to this accommodating member and having a terminal with a notched portion bent upward and a notched portion formed by the bending being provided on an upper surface of the accommodating member in a state not overlapped with the piezo-electric elements; an insulating plate formed having a planar shape corresponding to this electrode plate having a notched portion formed and provided on the upper surface of the electrode plate in a state where a terminal of the electrode plate protrudes upward from the notched portion; and a taking-out member connected to the terminal protruding from the notched portion of this insulating plate and for taking out an output of the piezo-electric elements.